The present invention relates to anodic thinning of an epitaxial layer in the forming of a semiconductor such as a field effect transistor, and more particularly to a protective mask and method of using the mask to prevent excessive thinning of the epitaxial layer.
Anodic thinning is an effective procedure for reducing the active layer thickness of epitaxial structures to the optimum values required for device or circuit fabrication. While the basic procedure is self limiting, it is capable of significantly improving the uniformity of nonuniform surface structures and has been applied to gross thinning of entire epitaxial wafers prior to device fabrication.
The gross anodic thinning of semiconductor devices is disclosed in U.S. Pat. No. 3,890,215, to DiLorenzo et al. An epitaxial layer on a semi-insulating substrate designed for use in a field effect transistor is subjected to successive electrolytic oxidations and dissolution of the oxide by a dilute acid until the depletion region resulting from the applied potential extends into the semi-insulating region whereupon oxide growth and anodic thinning stops. The thinning process is self-limiting at a desired thickness dependent on the potential applied, the semi-conductor, the doping level of the epitaxial layers, and the electrolyte.
When the DiLorenzo et al method is applied to localized thinning, as opposed to gross thinning, the process is no longer self limiting and continues beyond the desired optimum value resulting in a channel thickness unsuitable for device fabrication.